Allow storage of the design in formats which can be used in other CAD and CAM packages

• There are other features of great value to a CAD system,

Don’t forget the manuals, They can help get through the tough times. If not, you may spend a lot of money calling help lines.

15.4 Editing and Creating

• Each CAD package allows us to manipulate the geometrical model using various interactive techniques.

• Editing Geometry depends upon the representation the geometry is stored with.

• If an elemental (remember: lines, circles, arcs, etc.) geometric model is used, then the methods are much different than a B-Rep model.

• The major editing methods used are for Elemental, Surfaces, and CSG.

15.4.1 2D Curves and Lines

• A number of functions must be provided to allow editing of 2D geometrical entities, such as lines, circles, arcs.

• Some of the basic editing functions are listed below,

Basic Entity Creation (lines/ circles/ arcs/ etc)

create using exact coordinates

two screen points for line ends, circle radius/center/diameter/etc

Line Trimming

trim lines back to intersection

extend lines to intersection

trim line to perpendicular point

cut a circle/arc on one side of an intersection

Point Creation

screen position

exact numerical coordinate

nearest tangent of line to an arc

nearest end of a line

midpoint of nearest line

center of nearest arc

nearest grid point

Arc Creation

intersection of circle with another line

Special Techniques

offset of a line

extend lines to intersection

trim line to perpendicular point

delete entities

etc

• There are a number of ways (philosophies) for creating drawings using the basic elemental editing techniques. A few popular methods are listed below.

Construction Lines: A set of construction lines are set up, then segments of the lines are selected for the actual drawing

Trimming: The construction lines are all drawn, then the unwanted parts are trimmed off

Navigation: A line figure is built up using successive line segments.

Parametrization: Objects such as rectangles, circles, arcs, etc. are created using their dimensions, then positioned with traditional methods.

15.4.2 Surfaces

• Most surface modeling packages rely on the elemental definition of lines, and points.

• There are a number of basic philosophies for creating surfaces,

Swept profiles: a profile, and a path in space are used to sweep out a surface.

Rotated Profiles: a Profile is created then swept about an arbitrary axis

Extruded Profiles: a profile is created, then grown in one direction.

Skins (Splines): a direct creation of points, then the splines that connect them

Polygon Approximation: polygons are defined which join up to define a surface

Sections: sections are defined for different points along a path, which then allow generation of complex transition geometries.

• Once surfaces have been created, they may be operated on by boolean operations.

• This method is often used as a preliminary stage to CSG editing.

15.4.3 CSG

• This is by far the simplest method

• Solid Primitives are progressively cut and joined to form new shapes.

• Primitives may come from,

Traditional Sources: Blocks, Spheres, Wedges, etc.

Surfaces: A Volume is assigned to a surface model

Previous operations

others ?

• CSG editing requires storage of the results of operations. This is because a part may be used many times to cut another part, for example a chamfered hole for a sunken screw.

• The fundamental CSG operations are,

Union: both parts joined as one

Intersection: Only where two parts overlap

Subtraction: only where parts do not overlap, One of the parts is typically discarded.

15.5 User Interpretation of Geometric Models

• Every CAD system uses a graphical display for user interpretation of the final part.

• The display methods discussed in the computer graphics section are all used in CAD packages. (Please refer to that section for examples, and explanation of how each display method affects the user).

• There are many techniques possible with computer graphics that make on screen designs easier to understand.

Dimensioning

Placed manually, but updates when dimensions change.

Annotation: the user may add comments to drawings

text with a leader pointing to something

text alone

tolerances

Drawing information

Graphics effects

Fill Styles: hatching and other patterns selected

Line Styles: such as hidden, shadow, phantom, etc.

Color: helps differentiate when there are many lines

15.6 User Directed Changes to the Geometric Model

• This feature is of the greatest importance to a user

• Some onscreen input selection features are,

Boxes: a good example of this method is the zoom boxes

Types: all objects of a certain type are selected

Last: last object created

All: all visible objects

Names: in some systems parts are named, and these can be used for reference.

Markers: symbolic markers can be placed on the screen to allow easy differentiation between object (like in Ideas)

Layers: drawings can be layered up, which allows easy separation of distinctive parts. One example is a factory layout, including separate layers for machines, plumbing, electrical, HVAC, etc.

• Objects often require manipulation on the screen, and certain parameters defined by a number of techniques,

Translation

select two points (they define a translation vector)

angle and distance

select a line (a direction vector)

move a point to a point

interpolate

etc.

Rotation

about a pivot point by 1 angle (2 angles for 3D)

about an axis by an angle

Mirroring

mirror over a line/plane

Magnification

a magnification about a point by a factor

Copying

a simple copy

a copy with an offset for each copy

a copy with a rotation for each copy

15.6.1 Modern Hardware for CAD Systems

• The CRT is taken for granted now, but previously CAD systems underwent many metamorphosis.

• The array of current and previous hardware input devices are,

keyboard

mouse

tablet

button/dial boxes

light pen

touch screen

track balls

joy sticks

punched cards

CMMs

scanners

3D scanners

• Future hardware input devices include,

virtual reality gloves

voice

scanned input and recognition

vision systems

• Previous output devices include,

Text printers

Graphics printers

Plotters

CRT

Rapid prototyping

Virtual reality vision systems

15.7 Selecting a CAD System

• While this apparently seems easy, it is a very complex decision

• There are a number of factors which affect how the system is received,

Current computer use by employees

Perceived role of computer by employees

Cost of computer system and software

Available training for staff

Maintenance requirements for computer

Required number of users

Design complexity

Availability of CAM Facilities

Successful implementation in similar facilities

Management philosophy

Redundancy of design

• Major mistakes are,

Assuming more expensive is better

Assuming it will be well received because it will make work easier

Assuming that high tech means easier to use

Failing to get the potential users interested and involved in the decision to buy/selection/implementation

Neglecting the break in period

Not thoroughly examining the existing manual/computerized system which ALREADY WORKS

Forgetting that accountants want numbers plus a rate of return.

15.7.1 An Example Plan for Selecting a CAD system

1. Examine the existing situation. Involve staff to find out what they perceive as problems, and possible solutions. This establishes allies required for whatever decision you choose. (Expect some who will resist, but they can become allies if handled properly).

2. Identify key people with an interest in the system, and get them involved with selection.

3. Devise a definite list of requirements, to support existing functions, and problems which exist, and possible solutions.

4. Get the accountant on your side by consulting them about costs, budgets, etc.

Microsoft introduces Windows, bringing windowed interfaces to the last major computer platform.

• Some Concepts in GUIs are,

button: An item which is shown within a window. When a user points at it, and presses a mouse button, it initiates an action.

icon: A small graphical symbol on the screen which can be opened to expose a window

menu: A pop up menu which stays hidden until called up by mouse. This simplifies problems of crowded screens.

mouse: a very popular input device for graphics programs. The use can point and choose an item. Contemporary alternatives are track-balls, joy-stick, dial boxes, tablets, etc.

scrollbars: At this side of some graphical, and text windows are bars which can be used to move the window around, to see previous text, or hidden areas of a graphics screen.

slider: A bar chart type of input, where the user can use the mouse to pull the slider along, and change an input value

window: A panel for keyboard and mouse I/O, which can be layered on a screen with other windows, like paper on a desk. The user often selects to work in a specific window by pointing the mouse into it. A Window may be closed, to become an icon

• Popular window systems are (not a complete list),

OS/2: IBMs attempt to take control of the operating system used on the IBM PCs, and bring full capability to PC architecture.